Carbon-carbon composites, that is, carbonized material reinforced with carbon fibers consisting essentially of elemental carbon and some graphitic materials can be prepared in a wide variety of shapes exhibiting a number of unique properties. Importantly, such materials possess unusual resistance to high temperature environments, for example, they show thermal stability as a solid, and due to their high thermal conductivity and low thermal expansion behavior, they resist thermal shock. The materials also maintain a high degree of strength and stiffness when exposed to high temperatures. Such properties are explained by the high refractive nature of elemental carbon and by the high strength and stiffness of fibers formed from amorphous and graphitic carbon.
In view of the preceding, therefore, it is not surprising that carbon-carbon composites have found, and will continue in the future to find broad use in lightweight, aerospace applications. They are, for example, widely used in the field of space vehicle re-entry, and in this regard, the space shuttle employs carbon-carbon composites in its nose cone and wing edges. Disc brakes for high performance aircraft provide another application depending upon the unique properties of carbon-carbon composites.
While carbon-carbon composites possess many valuable characteristics, including those mentioned, they suffer from the disadvantage of being expensive to fabricate; thus their use is normally restricted to applications in which properties, rather than cost are of primary importance. The high cost of the composites is due to an important degree to their method of fabrication, which necessarily involves multi-step processing, as will be explained in the following.
In a typical procedure, the composites are formed by arranging carbon fibers in a two, three or four directional, occasionally unidirectional reinforcing structure, which is impregnated with a carbon-containing compound that functions as a precursor for the carbon matrix. Following impregnation, the structure is heated in a carbonization step conducted under an inert atmosphere to the decomposition temperature of the precursor, for example in an autoclave, thereby producing a more dense fibrous structure whose interstices are at least partially filled with carbon resulting from the pyrolysis of the precursor material. The structure thus processed is then removed from the autoclave, reimpregnated with additional matrix precursor and again subjected to a carbonization treatment, resulting in additional filling of the composite's interstices with carbon. The procedure is repeated, frequently as many as four to twenty times, until satisfactory composite densification has occurred.
Unfortunately, each carbonization cycle required to achieve the desired densification is both time consuming and labor intensive and, therefore, costly. Consequently, it will be appreciated that the precursors should have a high carbon or "char" yield, i.e., a low weight loss during the carbonization treatments. The use of precursors yielding a high char level assures the greatest possible yield of carbon from each carbonization cycle, thus minimizing the number of cycles required and the cost necessarily entailed in the process.
In addition to an ability to form relatively high amounts of carbon char, it is advantageous for composite matrix precursors to exhibit a low viscosity as well as the ability to wet the surfaces of the carbon substrates being impregnated. In this regard, viscous materials are undesirable since it is difficult to accomplish satisfactory penetration of such substances into the interstices of the carbon structure, particularly since the interstices grow smaller with each successive carbonization cycle. Furthermore, while precursors that flow readily during the impregnation portion of the cycle are of advantage since they facilitate penetration throughout the interior of the structure, the precursors should desirably resist flow during the carbonization process. This apparent anomaly is explained by the fact that liquid flow must be inhibited during the process of heating the precursor materials within the filamentary carbon structure to their decomposition temperature in order to avoid loss of the materials from the structure.
In an effort to provide superior matrix formation, various materials have been used in the past to form carbon-carbon composites. Coal tar pitch is among such materials, and while it is capable of yielding relatively high levels of char; it is a relatively viscous material, making the impregnation process difficult. In addition, high carbon yields from coal tar pitch are possible only if the carbonization process is performed very slowly or under relatively high pressure, for instance, in the neighborhood of 100 bars. Also, the viscosity of such pitches decreases with temperature, producing undesirable liquid loss during heat-up of the composite structure to the pyrolysis temperature of the pitches.
Precursors comprising thermosetting resins, on the other hand, do not need to be subjected to pressure during carbonization, and since they undergo cure prior to carbonization, they are not characterized by the disadvantage of tending to separate from the composite structure before precursor carbonization. Unfortunately, however, the char yield of thermosetting resins, which can in some cases be in the order of 39-51%, is not particularly high, necessitating relatively more carbonization cycles. Various other precursors have also been suggested and used in the past with varying results, but the search for precursors displaying optimal results in the formation of carbon-carbon composites has proceeded.